The present invention relates to a method of controlling a continuously variable speed transmission for controlling the running of a motor vehicle incorporating the continuously variable speed transmission.
One known method of controlling continuously variable speed transmission for controlling the running of a motor vehicle incorporating the continuously variable speed transmission comprises the steps of setting a reference engine rotational speed (referred to as a "running reference engine speed" or a "R-REF engine speed") corresponding to a throttle valve opening of the engine, and controlling the engine speed so as to coincide with the R-REF engine speed (see Japanese Laid-Open Patent Publication No. 62(1987)-237164, for example).
In the above known ratio control, when an accelerator opening (which may refer to the amount of depression of an accelerator pedal or the throttle pedal opening) is not fully closed because of depression of the accelerator pedal or the like, a R-REF engine speed may be set according to the accelerator opening for controlling the speed reduction ratio of the transmission. When the accelerator pedal is not depressed and the accelerator opening is substantially zero (this condition is brought about by applying engine braking for vehicle deceleration), if a R-REF engine speed is set according to the accelerator opening by the above control method, then the engine speed would be too low to obtain a sufficient engine braking power.
For applying an engine brake when the accelerator opening has become substantially zero, it has been conventional practice to set a reference engine speed (referred to as an "engine braking reference engine speed" or an "EB-REF engine speed") for obtaining a desirable engine brake, according to the running speed of the motor vehicle, for controlling the speed reduction ratio.
However, setting the EB-REF engine speed and the R-REF engine speed separately from each other for ratio control is problematic in that the controlling process is complex. Furthermore, when depressing the accelerator pedal while an engine brake is being applied to the motor vehicle thereby to enter a control mode based on the R-REF engine speed, or vice versa, the driver may feel uncomfortable since the reference engine speed is abruptly changed.